Nitrogen oxides (NOx), carbon monoxide (CO), and unconverted (unburned) hydrocarbons (HC) are toxic and environmentally damaging pollutants found in the exhaust gas from internal combustion engines. Catalytic converters are used to decrease these pollutants by oxidizing HC, soot, and CO to CO2 and water, and by reducing nitrogen oxides to N2. Both oxidation and reduction reactions must occur in catalytic converters in order to remove these pollutants in a single system. Most gasoline engines use a three way catalyst (TWC) system to remove CO, HC and NOx while diesel engines use a four way catalyst to convert those three pollutants plus organic particulates (PM).
For effective abatement of these pollutants, most catalytic converters utilize one or more platinum group metals (PGM) due to their efficiencies for catalyzing these oxidation/reduction reactions. With the ever stricter standards for acceptable emissions, the demand on PGM continues to increase, placing a strain on the supply of PGM, which in turn drives up the cost of PGM and therefore catalysts and catalytic converters. Thus, there is a need to develop advanced catalysts and methods of synthesizing these catalysts that utilize these PGMs in a much more efficient way to increase the longevity of the catalytic converters which will reduce their overall cost.
With changes in engine technologies, the catalytic materials used in gasoline and diesel converters have also changed. These engine advancements require new materials that are highly active and thermally stable under a wide range of exhaust gas conditions. To abide by the EPA exhaust regulations there is a need for new catalytic materials that operate (light-off) at lower temperatures to reduce exhaust pollutant emissions during vehicle start-up, particularly in cold temperatures. Start-up emissions may be reduced by moving the catalyst closer to the engine exhaust manifold, like close-coupled converters, but this results in very high catalyst temperatures under normal driving conditions that are detrimental to the conventional catalysts in these close-coupled converters.
For the foregoing reasons, it would be advantageous to provide a method for converting the pollutants from a reactant gas mixture of exhaust by supplying the reactant gas mixture to suitable catalysts. The suitable catalysts include mixed-metal oxides such as pyrochlores and brownmillerites, where the mixed-metal oxide crystal lattices are substituted. These substituted mixed-metal oxides along with the methods and systems incorporating them, provide conversion of pollutants using low amounts of PGM catalysts while possessing synergistic redox functions that yield enhanced activity and performance under both lean and rich engine conditions.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.